Pregnant Mares

The mare from which you have always wanted to raise a foal has just been pronounced pregnant by your veterinarian. Now you just have to wait about 11 months and see what she produces, right? Not so fast! There is much more involved than just

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The mare from which you have always wanted to raise a foal has just been pronounced pregnant by your veterinarian. Now you just have to wait about 11 months and see what she produces, right? Not so fast! There is much more involved than just waiting if you want the foal to have the best chance of being born healthy.


Your first concern is to maintain that newly detected pregnancy. Barry Ball, DVM, PhD, Dipl. ACT (reproduction specialist), University of California, Davis, showed early embryonic loss occurred at a rate of 9% in young, fertile mares by Day 14 after fertilization, with a loss rate more than 60% for aged, sub-fertile mares during this same time period. Therefore, rechecks of the pregnancy are a must so if an early loss occurs, re-breeding is an option.


Can you stop early embryonic loss? One possible defect, inadequate progesterone secretion, is a rare occurrence, but many breeders supplement progesterone or synthetic compounds with similar actions such as altrenogest (brand name Regu-Mate) during early pregnancy. Progesterone causes the cervix to remain tightly closed, the uterus to remain firm, and helps block production of prostaglandins (hormones that arise from inflammation in the uterus).


Why not give progesterone to all mares in early pregnancy? It is expensive, potentially harmful to handlers, and unnecessary in many cases. Progesterone deficiency can be documented by monitoring progesterone levels frequently during the first few weeks of pregnancy. Repeated early pregnancy loss is a reason for concern, and your veterinarian’s input is needed. Managing stress can offset this problem in many cases. Minimize stress by eliminating sudden feed changes, extremes in exposure to adverse weather, threatening dogs chasing the horses, loud noises, etc

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A. C. (Woody) Asbury received his DVM from Michigan State University in 1956, then spent 21 years in California in breeding farm practice and at UC Davis. He joined the faculty at the University of Florida in 1977 and was involved in teaching, research, and administration until 1996. Asbury was a long-time member of The Horse’s advisory board. He died in 2011 after a lengthy illness.

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